Good photos! Here's a break down on each and how they could be improved:
1) Exposure is good, but composition wise it could be improved. Lighting is a bit flat, but good.The subjects head is dead center in the frame. Having them slightly off center or on one of the left or right thirds (read up on "the rule of thirds) may make for a more interesting shot. Experiment with cropping and you'll see what I mean. Also, watch out for your own shadow. We can see you at the bottom of the frame!
2) Lighting is good. It seems that you missed your focus here. The column behind the subject is the focus point for this shot so your subject is out. Composition wise I'd crop the subject over to a third for a stronger image.
3) This is the strongest of the bunch! While the subjects head is dead center, her shoulder, head, and hair cover 2 thirds of the photo to give an off center composition. You also get some nice soft fill light and a nice subtle catch light in the eyes. Nice work! Maybe bring a reflector next time to fill in the shadows just a tad more. Also, the focus is slightly off. It looks like your AF focused on her hairs in front of her face so her eyes are a little soft. A narrower aperture may be better until you start nailing your focus. Try f4-f8 until you get the hang of it. The 50mm 1.8 also tends to get much sharper at these narrower apertures as well.
4) Lighting is good. Again, the subject is dead center. Try cropping and framing them to a third for a stronger composition.
Overall not bad, but definitely read up on "the rule of thirds" and composition. You may also want to dial back your noise reduction if these are JPEG's right out of the camera (the look a little soft and smudgy) and up your sharpness to about 2 or 3. I find that the default sharpness settings are a little soft when compared to RAW. At least that's what I find with the 7D, but the 60D is probably nearly identical. Hope this helps!
----------
Sorry forgot to add something. I'm not sure if you're saying that you used the same exposure, ISO and aperture settings for all these photos, but if that's the case you may want to try and use different settings for each shot. For example, the third shot could have used a slightly faster exposure and narrower aperture. Sure you'd need to go to a higher ISO, but not by much. Maybe instead of going into full Manual mode stick with Aperture priority for a while. Unless I'm in the studio or doing long exposures my camera stays on Aperture priority the majority of the time.
